Where do quasar hosts lie with respect to the size-mass relation of galaxies?

TL;DR

This study investigates the sizes of quasar host galaxies at redshifts 1.2-1.7, finding they are intermediate in size compared to star-forming and quiescent galaxies, providing insights into galaxy evolution and AGN feedback mechanisms.

Contribution

It provides the first detailed size measurements of AGN host galaxies at this redshift range, exploring their position relative to the galaxy size-mass relation and implications for galaxy evolution.

Findings

AGN hosts have sizes from 1 to 6 kpc at Mstellar ~ 0.3-1 x 10^11 Msun

Many AGN hosts have intermediate sizes compared to star-forming and quiescent galaxies

Results suggest AGN feedback may not significantly increase galaxy sizes

Abstract

The evolution of the galaxy size - stellar mass (Mstellar) relation has been a puzzle for over a decade. High redshift galaxies are significantly more compact than galaxies observed today, at an equivalent mass, but how much of this apparent growth is driven by progenitor bias, minor mergers, secular processes, or feedback from AGN is unclear. To help disentangle the physical mechanisms at work by addressing the latter, we study the galaxy size - Mstellar relation of 32 carefully-selected broad-line AGN hosts at 1.2 < z < 1.7 (7.5 < log M_BH < 8.5; L_bol/L_Edd > 0.1). Using HST with multi-band photometry and state-of-the-art modeling techniques, we measure half-light radii while accounting for uncertainties from subtracting bright central point sources. We find AGN hosts to have sizes ranging from 1 to 6 kpc at Mstellar ~ 0.3 - 1 x 10^11 Msun. Thus, many hosts have intermediate sizes as compared to equal-mass star-forming and quiescent galaxies. While inconsistent with the idea that AGN feedback may induce an increase in galaxy sizes, this finding is consistent with hypotheses in which AGNs preferentially occur in systems with prior concentrated gas reservoirs, or are involved in secular compaction processes perhaps responsible for simultaneously building bulges and shutting down star formation. If driven by minor mergers, which do not grow central black holes as fast as they do bulge-like stellar structures, such a process would explain both the galaxy size - mass relation observed here and the evolution in the black hole, bulge mass relation described in a companion paper.